Until now, silver nitrate has been produced by dissolving a metal radical in nitric acid. The silver nitrate thus obtained, however, contains various heavy metals (e.g., Fe, Cu, Pd, Ni, Hg, Cd, Pt, Rh, and Cr) as impurities. However, to use this silver nitrate in certain photographic, catalytic, and analytical applications, it is necessary to remove these heavy metal impurities.
Conventional methods for removal of heavy metal impurities from crude silver nitrate solution can be roughly classified into a crystallization method, an adsorption method, and a coprecipitation method.
Of these methods, the purification method utilizing adsorption is most advantageous for use with a process because purification can be carried out continuously. That is, heavy metals can be removed selectively and continuously by passing the crude silver nitrate solution through an adsorption column packed with granular adsorbent such as carbon black, activated alumina, and magnesia. Of these adsorbents, activated alumina is the only one that is not a fine powder, and thus least likely to clog the adsorption column. However, activated alumina is difficult to use because the rate of removal of heavy metal impurities varies markedly depending on the type of the metal.
Recent improvements in performance of photographic silver halide light-sensitive material require high purity silver nitrate as a main starting material.
This has increased the demand for the effective removal of heavy metal impurities from silver nitrate. Group VIII elements of the Periodic Table, especially the platinum group elements, which are harmful for photographic performance, must be removed so that no more than 15 ppb, preferably no more than 5 ppb, remain. Conventional activated alumina, however, does not have sufficient adsorption capability to meet these requirements.
U.S. Pat. No. 2,543,792, for example, discloses a method in which an aqueous silver nitrate solution is continuously brought into contact with elemental carbon (for example, carbon black) and activated alumina, and then treated with silver nitrate to remove metal impurities. U.S. Pat. No. 2,614,029 discloses a method in which aqueous silver nitrate solution is treated with sufficient silver oxide to make the pH of the aqueous silver nitrate solution at least 6.1; the metal and metal hydroxide thus precipitated are separated; and the resulting solution is brought into contact with a water-insoluble porous solid adsorbent such as activated alumina or magnesia to remove the metal impurities.
In accordance with the methods described in the examples of the above U.S. Patents, however, even though a combination of activated alumina and carbon black as an adsorbent is employed, the concentration of residual platinum group elements such as platinum (Pt), rhodium (Rh), and palladium (Pd) is 20 to 300 ppb.
As described above, conventional activated alumina used as an adsorbent does not have an adsorption capability sufficient to remove platinum group elements that are harmful to photographic performance, to the order of magnitude less than 15 ppb. For this reason, even though adsorption purification using activated alumina is used, crystallization must be repeated several times after the adsorption purification in order to remove the platinum group elements to an order of magnitude less than 15 ppb, or the adsorption method must be employed in combination with other methods. In repeating the crystallization, the heavy metal elements are concentrated in the filtrate, and thus the purification efficiency is increasingly decreased. Thus, the advantages of purification using adsorption cannot be attained.
Another method of removing the platinum group elements to an order of magnitude of 15 ppb is described in International Application JP-A-63-502890. In accordance with this method, (1) crude silver is dissolved in nitric acid to form a crude silver nitrate solution, (2) an alkali agent such as sodium hydroxide is added to the crude solution to precipitate metal impurities, and an incompletely purified silver nitrate solution is separated, (3) a selective reductant such as sodium formate is added to the solution to precipitate silver powder while leaving the metal impurities in the solution, (4) the silver powder is dissolved in nitric acid to form a highly purified silver nitrate solution, and (5) crystallization is conducted to obtain ultra-pure silver nitrate. This method is unsuitable for practical use because the operation is complicated. In addition, the production cost is high because twice the quantity of nitric acid, compared to that of other methods is required.
As described above, to produce high purity silver nitrate for use in photography, crystallization must be repeated many times after adsorption purification with activated alumina, or a method as described in International Application JP-A-63-502890, which is complicated in operation and requires a high production cost, must be employed.